The present invention relates to an ink-jet printer (printing apparatus) and a printing head driving method therefor and, more particularly, to an ink-jet printer for printing by scanning a printing head having a plurality of nozzles arranged in a predetermined direction, each designed to discharge ink droplets, over a printing medium in a direction almost perpendicular to the array direction of the nozzles, and a printing head driving method for the apparatus.
As an information output apparatus used for a wordprocessor, personal computer, facsimile, or the like, a printer is available, which prints desired information such as characters and images on a printing medium such as paper or film.
As printing schemes for printers, various schemes such as a dot-impact scheme, thermal scheme, ink-jet scheme are known. The ink-jet printing scheme is one of so-called non-impact printing schemes, and has the following advantages. The noise generated in printing operation is negligibly low. High-speed printing and printing on various recording media can be performed. Images can be fixed on even so-called plain paper without any special process. In addition, high-precision images can be obtained at low cost.
Owing to these advantages, printers using the ink-jet scheme have rapidly become popular in recent years as printers for copying machines, facsimiles, wordprocessors, and the like as well as printers serving as peripheral devices of computers.
Generally used ink discharging methods in the ink-jet printing scheme include a method of using electrothermal transducers (heaters) and a method of using piezoelectric elements. In either method, discharging of ink droplets is controlled by electrical signals.
According to the principle of ink droplet discharging operation using electrothermal transducers, when an electrical signal is supplied to a given electrothermal transducer, ink near the electrothermal transducer is instantaneously boiled (film boiling), and an ink droplet is discharged at high speed upon abrupt growth of a bubble produced by a phase change of the ink at this time. This method therefore has the advantages of, e.g., simplifying the structure of an ink-jet printing head and facilitating integration of nozzles.
In order to implement high-density printing, an ink-jet printing head often has a plurality of nozzles for discharging ink and discharge pressure generating elements. In general, a divisional driving scheme is employed, in which these nozzles are grouped into sections, each having a predetermined number of nozzles, in accordance with their physical positions, the nozzles in each section are further grouped into driving blocks, and the discharge pressure generating elements are time-divisionally driven in units of driving blocks. This divisional driving scheme is an effective scheme in achieving reductions in the sizes of power supply members such as a power supply for driving the printing head, a connector, and a cable.
In an ink-jet printing head using electrothermal transducers, in particular, variations in voltage value in a power supply for discharge pressure generating elements must be minimized, and the voltage value must be finely adjusted in order to implement stable discharging operation in consideration of the characteristics of the electrothermal transducer, ink, and the like. For this reason, a large power supply capacity is not preferable. The above divisional driving scheme is also effective in satisfying such requirements for a power supply.
A case wherein an ink-jet printing head is driven by the divisional driving scheme will be described in more detail below with reference to the accompanying drawings.
FIGS. 4A to 4C schematically show the nozzle array of the ink-jet printing head, driving signals for the respective nozzles, and flying ink droplets discharged from the respective nozzle, respectively. Referring to FIG. 4A, a nozzle array 500 of the ink-jet printing head is made up of, e.g., 32 nozzles, and these nozzles are grouped into four sections each having eight nozzles, from the first section to the fourth section, when viewed from the upper side of FIG. 4A.
In addition, each of the eight nozzles in each section belongs to one of eight driving blocks, and the nozzles are time-divisionally driven in units of blocks in printing operation. That is, the nozzles in the same block are simultaneously driven.
In the case shown in FIG. 4A, nozzles are periodically assigned to the respective driving blocks such that, for example, four nozzles, i.e., the 1st, 9th, 17th, and 25th nozzles of the nozzle array 500 are assigned to the first driving block, and 2nd, 10th, 18th, and 26th nozzles are assigned to the eighth driving block. The first to eighth driving blocks are sequentially driven in ascending order by pulse-like driving signals 300 shown in FIG. 4B, and ink droplets 100 are discharged from the respective nozzles in accordance with the driving signals, as shown in FIG. 4C.
Each nozzle has its unique characteristics associated with the discharge direction of ink droplets, the amount of ink discharged, and in the like. Such characteristics unique to each nozzle affect printed images, and may cause streaking, density unevenness, and the like. In order to eliminate such adverse influences on printed images, a multipass printing method is used, in which the ink-jet printing head is scanned over a printing area a plurality of times to print the same raster with two or more different nozzles.
An ink-jet printer is required to be kept in a state wherein ink can always be discharged stably. In some case, when ink is discharged by a discharge pressure generating element, variations in pressure due to the discharging of the ink vibrate the ink in an adjacent liquid channel through a common liquid chamber. If, therefore, the discharge pressure generating element disposed in the adjacent liquid channel is continuously driven, the pressure variations make discharging operation unstable, resulting in a change in ink discharge amount.
A change in ink discharge amount causes density unevenness in a printed image. Variations in ink discharge amount due to variations in ink pressure become more noticeable as the number of nozzles to be continuously and simultaneously driven increases. In addition, such variations are greatly influenced by the distances from the ink supply ports, the shape of the common liquid chamber communicating with the orifices, and the positions and sizes of residual bubbles in the common liquid chamber.
When the number of nozzles to be simultaneously driven greatly changes, the flow rates of ink into the liquid chambers vary. Such variations vibrate the meniscus surfaces of the nozzles through the common liquid chamber. As a consequence, discharging operation becomes unstable, and the amounts of inks discharged change, resulting in density unevenness in a printed image.
With regard to this change in discharge amount, experiments conducted by the present inventors confirmed that uneven density portions of a printed image depend on driving blocks. FIG. 5 is a graph showing driving signals for causing all the nozzles to periodically discharge ink droplets at predetermined intervals and the distances between meniscus surfaces and the orifices as functions of time. As shown in FIG. 5, with regard to driving blocks 1 to 3 belonging to the first half group, the meniscus position corresponds to a convex shape with respect to the orifice surface, whereas with regard to driving block 6 belonging to the second half group, the meniscus position corresponds to a concave shape with respect to the orifice surface. In this manner, each driving block has each specific meniscus state. This uneven pattern of meniscus directly corresponds the magnitude of discharge amount.
As described above, the method of time-divisionally driving is used for discharge pressure generating elements. In general, these elements are periodically arranged on a printing head substrate in a predetermined order. Owing to periodical ink pressure variations, therefore, periodic density unevenness occurs in a printed image. In printing an image by multipass printing, in particular, if a combination of driving blocks of nozzles for scanning the same raster is constituted by only blocks exhibiting large discharge amounts, density unevenness in a printed image becomes more noticeable.
In order to reduce such influences of changes in ink pressure in liquid chambers on printed images, the common liquid chamber is broadened or the physical distances between adjacent discharge pressure generating elements and time intervals at which the elements are driven are increased. This makes it difficult to attain a further reduction in printing head size and a further increase in printing speed.
It is, therefore, an object of the present invention to provide an ink-jet printing apparatus which can reduce density unevenness of a printed image due to periodic ink pressure variations, and a printing head driving method for the ink-jet printing apparatus.
According to the present invention, the above and other objects are achieved by an ink-jet printing apparatus for printing by scanning a printing head having a plurality of nozzles arranged in a predetermined direction, each designed to discharge an ink droplet, over a printing medium in a direction substantially perpendicular to an array direction of the nozzles, comprising driving means for time-divisionally driving the nozzles at timings of a plurality of blocks, wherein when multipass printing is performed by scanning different nozzles over each printing area a plurality of times, the driving means drives nozzles used to print the same raster at timings of at least two different blocks.
In addition, the above objects are also achieved by a printing head driving method for an ink-jet printing apparatus for printing by scanning a printing head having a plurality of nozzles arranged in a predetermined direction, each designed to discharge an ink droplet, over a printing medium in a direction substantially perpendicular to an array direction of the nozzles, comprising the driving step of time-divisionally driving the nozzles at timings of a plurality of blocks, wherein when multipass printing is performed by scanning different nozzles over each printing area a plurality of times, the driving step comprises driving nozzles used to print the same raster at timings of at least two different blocks.
According to the ink-jet printing apparatus of the present invention, in a driving scheme of time-divisionally driving the nozzles at timings of a plurality of blocks in multipass printing operation in which printing is performed by a plurality of times of scanning, nozzles used to print the same raster are driven at timings of at least two different blocks.
Proportionally distributing driving blocks used to print each rater in this manner can compensate for variations in printing density due to the differences in ink discharge amounts which are dependent on the respective printing timings, thereby reducing density unevenness. More specifically, high-density printing and low-density printing are alternately performed, and each raster is printed by four printing operations in an overlapped state. As a consequence, density differences among the rasters are canceled out, and each raster has an almost uniform average density, thus improving print quality.
The driving means preferably drives the nozzles used to print the same raster at timings of different blocks.
More specifically, when numbers are assigned to the blocks in a driving sequence, the numbers of the blocks for driving the nozzles used to print the same raster are preferably constituted by a pair of a number belonging to a first half group and a number belonging to a second half group.
When numbers are assigned to the blocks in a driving sequence, the sum of the numbers of the blocks for driving the nozzles used to print the same raster preferably remains unchanged among the respective rasters.
In addition, the number of times of scanning in the multipass printing operation is preferably an even number.
The number of blocks is preferably a number obtained by dividing the number of nozzles by the number of times of scanning.
The numbers of the blocks for driving the nozzles used to print the same raster may be complementary to each other between two adjacent rasters.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.